Review
High efficiency, actively Q-switched Er/Yb fiber laser
A. Gonza´lez-Garcı´a
a,b,n, B. Ibarra-Escamilla
a, O. Pottiez
c, E.A. Kuzin
a, F. Maya-Ordon ˜ ez
a, M. Dura´n- Sa´nchez
d, C. Deng
e, J.W. Haus
e, Peter E. Powers
eaInstituto Nacional de Astrofı´sica, O´ptica y Electro´nica, Departamento de O´ptica, Luis Enrique Erro No. 1, Sta. Ma. Tonantzintla, Pue., C.P. 72824, Me´xico
bInstituto Tecnolo´gico Superior de Guanajuato, Carretera Guanajuato-Puentecillas km. 10.5 Predio El Carmen C.P. 36262, Guanajuato. Gto., Me´xico
cCentro de Investigaciones en O´ptica, Loma del Bosque No. 115, Col. Lomas del Campestre, Leo´n, Gto., 37150, Me´xico
dUniversidad Tecnolo´gica de Puebla Antiguo Camino a La Resurreccio´n N 1002A Parque Industrial, Puebla, Pue. 72300, Me´xico
eElectro-Optics Program, The University of Dayton, 300 College Park, Dayton, OH 45469, USA
a r t i c l e i n f o
Article history:
Received 31 May 2012 Received in revised form 6 October 2012
Accepted 15 October 2012 Available online 15 November 2012 Keywords:
Actively Q-switched
Erbium and Ytterbium doped fiber Q-switched fiber laser
a b s t r a c t
We report the efficient and stable operation at 1549 nm of an actively Q-switched fiber laser based on an Er3 þ/Yb3 þ doped double-clad single mode fiber. It operates with a repetition rate from 45 to 120 kHz and pulse duration from 34 to 80 ns; the output pulse shape and peak pulse intensity are stable over hours of operation. For a repetition rate of 120 kHz and a maximum pump power of 8.1 W we obtained an average output power of 4.0 W with an overall efficiency of 50% and with a minimum pulse duration of 34 ns.
&2012 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . 182
2. Experimental setup . . . 183
3. Experimental results and discussion . . . 184
4. Conclusion . . . 186
Acknowledgments . . . 186
References . . . 186
1. Introduction
Q-switched fiber lasers have found widespread applications in laser trimming, marking and welding of various solid materials.
These lasers are characterized by short pulse duration and high peak power of the output pulses. Many techniques have been demonstrated for Q-switched lasers[1], but there are many more applications covering other fields of science and technology that require different laser characteristics (beam quality, system compactness and quantum efficiency). Laser systems based on
double clad, rare earth-doped fibers are attractive for compact and very efficient high power and short pulse generation. Their main performance advantage, compared with conventional bulk solid state lasers, results from the combination of beam confine- ment and excellent heat dissipation. Mostly Yb-doped high power systems are reported. Nevertheless the development of new fiber optic laser systems for eye save applications is of great interest for specific applications especially for atmospheric transmission in free space optical communication and active remote sensing, medical applications[2], and spectroscopy[3]. In some cases a longer wavelength is better suited for phase matching nonlinear conversion to longer wavelengths.
Many techniques are available for Q-switched laser operation;
for example passively Q-switched lasers were demonstrated using an external cavity configuration containing elements with a Co2 þ/ZnS crystal as a saturable absorber [4], and polymer Contents lists available atSciVerse ScienceDirect
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Optics & Laser Technology
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http://dx.doi.org/10.1016/j.optlastec.2012.10.021
nCorresponding author at: Instituto Nacional de Astrofı´sica, O´ ptica y Electro´- nica, Luis Enrique Erro No. 1, Sta. Ma., 72840 Tonantzintla, Puebla, Mexico.
Tel.: þ52 477 700 77 65; fax: þ 52 222 2472940.
E-mail addresses: [email protected] (A. Gonza´lez-Garcı´a), [email protected] (B. Ibarra-Escamilla).
composites have already demonstrated saturable absorber proper- ties on including carbon nanotubes [5] and graphene [6] in the matrix.
Novel active Q-switch devices[7–11] were proposed to pro- vide pulsed laser operation; for example a fiber Bragg grating was used to eliminate the need for bulk-optics components [8], yielding Q-switched pulsing at MHz frequencies. A single-fre- quency, all fiber laser at a wavelength of 1550 nm was demon- strated with a reported peak power of 25 W and a pulse width of 12 ns (0.3
m
J/pulse) at a repetition rate of 80 kHz[9]. In several publications, Q-switching devices based on MOEMS[10,11] tech- nologies were reported. On the other hand, acousto-optics mod- ulators (AOMs) [12] are commonly available devices used to produce actively Q-switched lasers and they have excellent damage thresholds, good extinction ratio, and stable operating characteristics that are not available in other devices.For cladding pumped fiber laser operation, the Erbium absorp- tion is low. Doping the fiber with high Er3 þ concentration is limited by ion–ion interactions that lead to dimer formations that degrade the pump efficiency. This problem has been solved by co- doping the fibers with Yb3 þ ions. Ytterbium absorbs the pump energy and transfers it to the Erbium ions. Er3 þ/Yb3 þ co-doping technique allows pumping of Yb3 þions using broad-stripe, high- power sources to reach much higher output power levels. Yb3 þ ions exhibit a strong absorption centered at a wavelength of 980 nm, but they can be pumped over a wavelength range extending from 800 to 1110 nm. Yb3 þexcited ions then transfer their energy to Er3 þ ions. The absorbed photons facilitate the transfer of population between the2F7/2level and the2F5/2level of the Yb3 þ ion. A cooperative energy transfer process between the excited state of Yb3 þ and the ground state of Er3 þ in the4I15/2
level excites the Er3 þto the4I11/2level while the Yb3 þreturns to its ground state. Much higher pump absorption efficiency is achieved this way[13–16]. Previous publications on the operation of high power lasers show that an important aspect is doped-core design[17,18]. The highest energy per pulse in a fiber laser was reported using a large mode area fiber for a high brightness output beam[19]. In this work, we exploit the Q-switched single mode operation to increase the peak intensity by designing a stable pump configuration that produces high average output power and short pulses using an Er3 þ/Yb3 þco-doped double clad single-mode fiber and we minimize heating of the fiber end.
We report stable Q-switched operation of a single-mode fiber laser with high pump slope efficiency operating at a wavelength around 1.5
m
m. The main motivation for our work is its potential applications in coherent laser radar and tunable sources for down-conversion, for example to develop tunable THz sources with mW output power. In the next section we present the experimental setup and examine the operational design charac- teristics of our Q-switched fiber laser. The laser was operated under varying conditions and the results are presented and discussed inSection 3. InSection 4we conclude by summarizing our most relevant results.2. Experimental setup
A schematic diagram of the actively Q-switched Er3 þ/Yb3 þ co-doped, double-clad fiber laser is shown inFig. 1. The optical setup is a combination of two optical sub-systems operating at different wavelengths. One of them is the pump system for 980 nm and the other one is the laser system for 1550 nm. The arrangement consists of an end pumped Fabry–Perot cavity with an intra-cavity acousto-optic Q-switch element. The dichroic mirror (DM2) and Sagnac loop mirror are employed as cavity end mirrors. The Sagnac loop mirror consists of a fused fiber coupler, in our case a 3 dB coupler whose output ports are spliced to 50 cm of Corning SMF-28 fiber, forming a short loop. This is a cheap and rugged device and the coupling ratio
a
of the coupler can be set to produce any desired reflectivity R. The properties of the Sagnac loop mirror were investigated in detail in[20]. In this laser the fiber loop is a convenient all-fiber mirror. The gain medium of the laser is a 3 m length of Er3 þ/Yb3 þ co-doped, double clad fiber, which has a core diameter of 7m
m, an inner cladding diameter of 130m
m, and an outer cladding diameter of 245m
m. The numerical aperture for the signal is 0.17 and the inner cladding to the outer cladding ratio is 0.46. The end of the Er3 þ/Yb3 þ doped fiber was spliced to a 1 m length of Corning SMF-28 fiber, which is single mode at 1550 nm, in order to attenuate the residual pump signal and also the 1064 nm signal due to the Ytterbium emission. The optical components in the system were carefully optimized through optical design software (ZEMAX) to reduce the spherical aberrations caused by the large numerical aperture of the fibers. Two aspherical lenses, L1 of 18 mmEr/Yb
DM2 HR@1550 nm HT@1064 nm AOM
SMF-28
Pump laser @ 976 nm
L2 L1
DM1 HR@1550 nm HT@1064 nm
RF driver
First order mode L3
3dB Coupler
Sagnac Loop mirror
3dB Coupler Output pulses @ 1549 nm
Fig. 1. Schematic diagram of the actively Q-switched Er3 þ/Yb3 þdoped double clad fiber laser. The elements in the design are discussed in the text.
focal length and L2 of 8 mm focal length, were used to efficiently relay and focus the pump and signal into the Er3 þ/Yb3 þco-doped, double-clad fiber. A lens L3 with a focal length of 150 mm was used in the system to make the beam diameter smaller than the Q-switch crystal size and at the same time to increase the stability of the cavity. The experimental setup includes two short wave pass (SWP) dichroic mirrors DM1 and DM2 with high reflectivity (499.5%) at 1550 nm and high transmission (490%) at 1064 nm, at 451 and 01 incident angles, respectively. The Er3 þ/Yb3 þco-doped, double-clad fiber was pumped by a high power diode laser (JOLD-30-FC-12- 976); the diode laser has its output fiber coupled with a core diameter of 200
m
m and numerical aperture of 0.22. The pump beam was coupled into the doped fiber through a 7.51 cleave at one end of the doped fiber. In experiments pump power was limited to a maximum pump power of 8.1 W.To perform Q-switching an acousto-optic modulator (AOM) with a diffraction efficiency of 460% at first order was inserted in the cavity. A 5 V transistor–transistor-logic (TTL) variable repeti- tion rate and duty cycle electrical signal was applied to the AOM driver. Several instruments were used to characterize the laser.
To measure the output power of the Q-switched laser we used a power meter at a second 3 dB coupler that was inserted close to the Sagnac loop mirror in the cavity, an InGaAs photodetector with a spectral range from 800 to 1700 nm and a bandwidth of 1.2 GHz, and an oscilloscope of 100 MHz analog bandwidth with a sampling rate of 1.25 GS/s were used to measure the pulse shape;
a monochromator was also used to measure the optical spectrum of the output signal; these measurements were made simulta- neously. The DM2 was adjusted to the position of the waist of the lasing signal. The distance between L3 and DM2 was very close to 230 mm, in fact much longer than the focal length of 150 mm for L3.
This is necessary because both pump and lasing signal with different wavelengths share the same lens L2. Fine-tuning of DM2 position guarantees a better coupling of the lasing signal into the core at the extremity of the doped fiber, forming a good cavity.
3. Experimental results and discussion
The spontaneous emission profile of our Er3 þ/Yb3 þ fiber covers the wavelength range from 1520 to 1570 nm. This profile of the Er3 þ/Yb3 þfiber allows a laser to operate within this region depending upon the cavity configuration.Fig. 2shows a spectrum measured with the monochromator in which we observe a strong peak at 1535 nm, in agreement with data sheet specifications.
The fiber length is a critical parameter in a laser. A laser with a longer Er3 þ/Yb3 þfiber length will operate at a longer wavelength
due to the preferential re-absorption of the shorter wavelengths in the un-pumped section of fiber. We measured the average output power for six lengths of the Er3 þ/Yb3 þ doped fiber.
The result is shown inFig. 3.
1520 1530 1540 1550 1560 1570
-40 -30 -20 -10 0
Intensity (dBm)
Wavelength (nm)
Fig. 2. Spontaneous emission spectrum of the Er3 þ/Yb3 þco-doped double-clad fiber.
0 2 4 6 8 10 12
0 1 2 3 4 5
Average Power (W)
Er/Yb fiber length (m)
Fig. 3. Variations of the average output power of the Q-switched laser with the Er3 þ/Yb3 þco-doped double clad fiber length.
0 2 4 6 8 10 12
1548 1552 1556 1560 1564 1568
Wavelength (nm)
Er/Yb Fiber Length (m)
Fig. 4. Variation of the wavelength of the Q-switched laser with the Erþ3/Ybþ3 co-doped double clad fiber length.
0 2 4 6
0 2 4 6 8 10
Signal Power (W)
Fiber Length (m) 1 W 0.1 W 0.01 W 0.001 W 0.0001 W
Fig. 5. Variation of the signal power along the amplifier length for various input powers.
It can be seen that there exists an optimal doped fiber length of 3 m for which we obtain the maximum average power from our laser; away from that optimal length the average output power falls off rapidly as shown in Fig. 3. The initial increase of the average output power with Er3 þ/Yb3 þ co-doped fiber length is due to the increase in the magnitude of the stored energy in the
fiber. Beyond the optimum fiber length a drop in average output power occurs due to the re-absorption of the signal from the un- pumped section of the fiber. Increasing of Er3 þ/Yb3 þdoped fiber length causes the operation wavelength shift to longer wave- lengths as shown inFig. 4. We can observe that for fiber lengths shorter than 3 m, the wavelength of operation of the laser is stabilized at 1549 nm.
An analytical analysis for Ytterbium fiber amplifiers was previously published[21]; we adopted that analysis to calculate the optimum doped fiber length and saturation power for our pumping scheme. Using the results presented in that reference we can demonstrate that the optimum fiber length is around 3 m for input signal powers from 104 to 1 W at constant pumping power (seeFig. 5). We can observe that for a length longer than 3 m, the gains are saturated for any value of the pump power.
Both theoretical and experimental results show that the 3 m length of Er/Yb fiber can be considered as an optimum length. The variations of the output power and pulse duration were measured as a function of repetition rate and the results are shown inFig. 6.
These results were obtained experimentally (using 3 m of fiber as an optimum length). For this length the output power starts to increase at repetition rates above approximately 80 kHz as shown in Fig. 6. Shorter pulse duration can be achieved with shorter lengths of fiber but at the expense of output power. The laser operation is optimal for repetition rates from 40 to 140 kHz; pulse durations were obtained between 80 and 34 ns. Once the laser operating conditions are determined the pulse train exhibits small fluctuations in peak power over hours of operation.
The output power of the laser increases for high repetition rates because the time-averaged excited state population is reduced, which decreases the energy lost due to spontaneous emission.Fig. 6shows that the Er3 þ/Yb3 þ co-doped double clad fiber allows high output powers to be maintained at high repetition rates because the pump light absorbed by Ytterbium ions is transferred to the adjacent Erbium ions by a fast cross- relaxation process; hence the Erbium ions experience higher pump intensity than in the absence of Ytterbium. This ability of actively Er3 þ/Yb3 þ Q-switched fiber lasers to operate at high repetition rates makes them suitable for applications which require high repetition rate. The average output power at a repetition rate of 120 kHz for several pump powers is plotted in Fig. 7. The line drawn shows that the lasing threshold is 3 W.
An average output power of 4.0 W at a wavelength of 1549 nm was obtained for a pump power of 8.1 W; this corresponds to a laser efficiency of 50%.Fig. 8shows a train of pulses and a single pulse of the Q-switched laser using 3 m of doped fiber.
40 60 80 100 120 140
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Repetition Rate (kHz)
Output Power (W)
20 30 40 50 60 70 80 90 100
Pulse Duration (ns)
Fig. 6. Variation of average output power and pulse duration with repetition rate.
0 2 4 6 8
0 1 2 3 4 5
Pump Power (W)
Average Output Power (W)
Fig. 7. Average output power against pump power into the fiber for 3 m span of Er3 þ/Yb3 þco-doped, double-clad fiber laser at the repetition rate of 120 kHz.
-300 -200 -100 0 100 200 300
0.0 0.1 0.2 0.3 0.4
Intensity (a.u)
Time (ns)
-20 -10 0 10 20
0.00 0.05 0.10 0.15 0.20 0.25
Intensity (a.u)
Time (ms) Time (ms)
Fig. 8. (a) Scope trace of the pulse train from Q-switched Er3 þ/Y3 þco-doped, double-clad fiber laser operating at 120 kHz, (b) profile of a single pulse.
One of the theoretical models used to calculate the pulse energy is reported in Ref. [22]which provides an approximate expression to calculate the maximum energy of a Q-switched fiber laser. The maximum energy is expressed as a function of several variables as
E ¼ Pð CW
t
21þnthhvVÞ1e1=ðt21frÞð1Þ where PCW is the output power in CW,
t
21 is the upper level lifetime of Er3 þ, nth is the population inversion at the laser threshold, h is Planck0s, constant, v is the frequency, V is the gain volume, and fris the repetition rate. The population inversion nthis given by
nth¼log Rð 1R2Þ þNEr
s
aErL2Lð
s
eErþs
aErÞ ð2Þwhere R1and R2are the reflectivities of the end mirrors, in this case the DM2 and Sagnac loop mirror,
s
aEr ands
eEr are the absorption and emission cross sections of Er3 þ, NEris the Er3 þ concentration and L is the cavity length. The one remaining issue to be addressed is to estimate the pulse duration, but a reasonable estimate can be obtained by dividing the energy by the maximum power:DtEE/Ppeak. For the following parameter choices: PCW¼ 1.5 W,t
21¼11 ms, fr¼120 kHz, R1¼99.5%, R2¼100%,s
aEr¼7 1025m2,s
eEr¼6.5 1025m2, l¼1549 nm, L¼5 m, NEr¼ 1.716 1025m3, we obtain a theoretical value of the pulse energy around 33.5m
J that is in good agreement with the experimental result of 34m
J (this value was obtained fromFig. 6).4. Conclusion
In conclusion, we used an Er3 þ/Yb3 þ co-doped, double-clad, single-mode fiber as the gain medium and an AOM as Q-switching element to setup a Q-switched fiber laser. Optical design software Zemax was used to choose the focal distance of aspherical lenses for maximization of the pump and signal coupling. The average output power of the laser was 4.0 W at maximum pump power of 8.1 W, showing an overall efficiency of
50%. For a pulse repetition rate of 120 kHz, laser pulses with pulse duration of 34 ns were obtained.
Acknowledgments
B. Ibarra-Escamilla was supported by CONACyT Grant No. 104551 and A. Gonza´lez-Garcı´a was supported by CONACyT Grant 175708.
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